New try. Perhaps it's in last Qt 5.15 version

src/core/ldapdn.cpp

@@ -46,7 +46,7 @@ bool LdapDN::LdapDNPrivate::isValidRDNString(const QString &rdn) const
         }
         return true;
     }
-#if QT_VERSION < QT_VERSION_CHECK(5, 15, 0)
+#if QT_VERSION < QT_VERSION_CHECK(5, 15, 2)
     const QVector<QStringRef> components = rdn.splitRef(QLatin1Char('='));
 #else
     // Split the rdn into the attribute name and value parts